And accidantially creating a new device not
available in
consumer market!
PDF-Version
If
you ever heard good music with headphones like a top-line STAX or
Sennheiser
model, than there is no doubt: you will never go back hear your
favorite music
like the Le Sacre du Printemps or The
Death and the Maiden or Like the Way I Do with
any other
headphone! All smaller speakers are simply less good (even the 300 Euro
ones)
or even extremely bad (the 30 Euro ones). Unfortunately really all the
high-end
headphones are designed for home-equipment connection. Some like the
Stax even
need full load-speaker drive and a special transformer. Most others
like the
Sennheiser, Philips, Beyer, etc. have a much higher impedance (the
famous HD 650
has 300W instead of the usual 30 to 60W - look at http://reviews.cnet.com/headphones/sennheiser-hd-650/4505-7877_7-30840714.html
or http://www.headphone.com/guide/by-manufacturer/sennheiser/sennheiser-hd-650.php
) and need therefore a larger voltage drive. So the question is how to
provide
a high-quality high voltage-level signal to your headphone if you are
mobile?
PC sound-cards, mp3-players, etc. usually have not
enough driving voltage for high-impedance phones, so just an
additional amplifier is needed. This means, in principle you can
connect such
high-impedance phone to your mp3-player, but it will be simple not loud enough to fully enjoy the
music. Of course, such audio power amplifier (PA) should be:
Unfortunately
there is no real commercial solution available! For a compact amplifier
an integrated audio power-amplifier should
be used, and luckily many are available for low prices, e.g. Texas
Instruments
is a leading manaufacturer. Discrete solutions can achieve even better
sound
quality, but are much harder to design and to build, and the
potentially better
quality may also come only if you pay
for it with: larger batteries, larger power consumption, larger size,
etc.
Unfortunately most older PA IC's are not really suited for high-end
applications (e.g. having a total harmonic distortion THD beyond 0.5%
and no
real rail-to-rail output, i.e. wasting a significant part of the supply
voltage) and require a large supply voltage (like 6V, 9V or even 12V),
which
makes the design bigger due to larger battery size for a given
capacity. Newer
chips can easily achieve THD below 0.1%. They typically work at 5V
(default USB
supply), which unfortunately makes the generation of large voltage
drive level
harder.
So
we have the dilemma that new chips are low-power, high-quality but
voltage-limited, and older ones (like the LM386) have lower sound
quality and
need larger batteries. One workaround is using bridge-load connection
(BTL),
because this doubles the voltage
output and allows high sound-levels even at 4.5V supply. Also BTL
allows
removing the usual big coupling capacitors at the outputs even with a
single
supply voltage.
So
I planned for this configuration using the Texas Instruments chip
TPA6021A4
which has the advantage of a conventional 2.54mm pitch DIL package. THD
is
0.07% at 1kHz even at 8W load and 20dB gain, so for 280W (measured at my HD 650) and 10dB at will be
even better (maybe 0.02…0.01%). Such DIL20 package is easier to solder
than
more modern SMD packages and the area penalty is minimum compared to
the
battery size needed anyway. For headphone-drive no heatsink is needed,
because
the theoratical maximum load current would be around 4V/280W=14mA, which is not much
larger than the low bias current of 7.5mA. This means also that for
high-impedance headphones the PA works in a near-Class-A mode, which is
best
for sound quality. Assuming a battery capacity of 900mAh (good Alkali
cell), we
can expect a hearing time of approximately 30 hours or more, good
enough for a
travel week. One positive side-effect is that the mp3-player playing
time will
increase by our PA, due to high input impedance (app. 20kW compared to e.g. 50W).
One
further advantage is that in BTL the distortion is smaller than in
single-ended
configuration (for same output current), because in BTL some harmonics
cancel
each-other (SE operation leads to typical THD of 0.3% using the
TPA6021, a pure
SE device TPA302 achieves appr. 0.1% - depending on gain setting).
So
this could be the circuit, which is very close to the TI application
recommandations:
Mainly
I made the caps at supply smaller for space reasons and due to much
lower currents
in the circuit (compared for load speaker application with 8W loads).
Supply
voltage can come from 3 small AAA batteries (also called LR03 or
micro). Also a
VDD switch is added to switch the PA fully down. The 100kW-poti allows gain adjustment
and sets the maximum output-level for given input. It can be adjusted
so that
only some distortions occur if you hear at maximum level. If you don’t
want to
have any adjustment, just replace the poti by a fix voltage divider
feading
app. 3V to pin 18 (e.g. 100kW to ground and 47kW to VDD to set the gain to 10dB). Note: In
principle you can set the
gain also higher (up to 20dB with pin 18 at VDD), but this has to be
paid with
a bit worse signal-to-noise ratio SNR, higher lower cut-off frequency
and
higher chance for undesired PA overdrive. The 10dB has been found more
or less by
trying, it is interestingly very close to the difference in sensitivity
between
the HD 650 and a typical in-ear earspeaker (sensitivity usually given
at
1Vrms).
After
some more thinking one overlooked problem comes up: Normal headphones
have a
standard stereo 6.3mm connector with shared
ground pin for left and right channel. That makes stereo BTL-connection
impossible (look into the schematic!).
Unfortunately the other solution of using a higher DC supply voltage,
would
allow only the usage of more power-consuming PA's and/or older ones
with less
good audio performance!
Luckily
the solution is simple: Like in an airplane the stereo connector can be
replaced by 2 mono connectors, which requires some simple soldering at
the
headphone connector. For Sennheiser headphones simply an additional
headphone
wire can be ordered (price around 15 Euro). Or just take the advantange
of the
any too long wires for mobile sound.
One
extra hint on assembly and component choice: The elements are quite
cheap,
alltogether maybe even below 30 USD, BUT better do not choose the
cheapest ones
or older components you may already have. Bad mechanics is the death of
all
electronics, and especially bad audio connectors can cause big trouble!
Also
the pcb should be a good one, easy to solder. I have used a fully
standard
breadboard-style pcb with manual wiring, best for prototyping. 1st
I
routed the ground net, then VDD, last the others. Use a text marker in
the
schematic to check all net connections already done. After soldering
and visual
inspection, check with a DMM for short-circuits (e.g. between ground
and VDD or
between neighboring IC pins) and for open-circuits. Then place the PA
to the
socket, probably some careful bending of the pins is required to make
it
easier.
For
the on-off switch better choose a rock-solid one which also indicates
if it is
close or not. For the PA itselfs I have used a standard high-quality
socket,
because that also reduces highly the risk of damaging the chip (by
over-heat or
ESD). If you have enough space, better use 6.3mm connectors than the
more
typical 3.5mm ones for less trouble. Also make shure that all wirings
and
solderings are reliable and get no mechanical stress. For the input I
have
directly soldered a cable with stereo 3.5m plug to minimize mechanics,
this way
achieving a direct connection to the headphone output of my mp3 player
(ZEN
from Creative – very nice piece!).
On
the pcb I put also a 3.5mm socket connected to the inputs to allow
driving a
normal headphone directly from mp3-player, i.e. without the amplifier
active.
Good if someelse wants to hear the music with a 2nd
headphone – of
course with no extra-amplification.
After
problems with weak contacts at the 2 output mono-connectors, I decided
to cut
the cable and to solder the ends directly to the board!
If
everything works 100% fine (if you follow all instructions – there is a
good
chance!), then it might be worth a decision to fully put glue over the
construction, just because the risk of too much mechanical stress is
much
higher than of electrical dammage and need for a repair.
Making
it smaller?
Good idea! If you
look at the construction pictures, you will
clearly see that the battery space dominates, also the box might be
smaller
(especially thinner). So option one might be using a smaller battery.
However,
note you need to stay between 4 and 5.5Volts (the TPA6021 would be
damaged
beyond 6V)! So this is clearly a possible way, although it may lead to
the need
for non-standard batteries and - of course - to reduced operating time.
Another
option is to create a more compact board, e.g. by using SMD components
and a
smaller on-off switch. A suitable SMD PA would be the Texas Instruments
TPA6020A2. This has has also the advantage of working down to 2.5V, so
it would
use really the full battery capacity in an optimum way. Great would be
a fully
optimized box, best with integrated easy-to-open battery part. Removing
the
optional input 3.5mm connector could also save space, and this function
could
be achieved - if needed - with an external standard adaptor.
Some
sound quality remarks
Of course, as
home-equipment PA's typically operate with
supplies of e.g. 60V, instead of 4.5V, they are able to deliver even
higher
sound levels than this PA and also a slightly better quality. However,
due to
BTL our PA delivers a sound pressure level - e.g. with the HD 650 -
really beyond the one people should accept for
longer duration! Also the Texas Instruments TPA6021A4 is definitely
better than
most other integrated PA within mp3-players, etc. As we have now
removed the weakest point in the signal chain (just
the standard-quality headphones), now for ultimate mobile sound quality
also other factors become more important such
as mp3-datarate (with top-quality headphones you will often find that
128kBit/s
is not good enough, suprisingly modern or classical music might be not
as
critical as older records, just because of artefacts) or mp3-player/PC
built-in
digital-to-analog converters (which are not as good as more
power-hungry DAC's
adressed to home-equipment).
BTW,
such amplifier we described here is not available as commercial device
– even
not a similar one! Just one example where soldering your own circuits
still
makes sense (and some fun)!
Alternatives?
Using
e.g. only two batteries, but getting a large output voltage is possible
by
using a DC-DC boost converter. Unfortunately such solution requires
more
external elements, and available fully integrated solutions (LM48824 or
TPA6140) do not yet adress high-performance applications and
high-impedance
speaker drive.
Really
no commercial solutions?
I found e.g.
companies like Boostara (see http://www.boostaroo.com)
and IASUS ( http://www.iasus-concepts.com),
which
offer some headphone amplifiers. However, they do not offer any
portable
product for driving a 300W load with 8Vpp – as we do here. This is yet
only available for 12V or
AC-powered amplifiers. A pretty good device is the Total AirHead from
HeadRoom
(99 USD) see http://www.headphone.com. It can deliver at least 4.5Vpp
by using
4 batteries (just because it uses no BTL), so this device is much
heavier and a
bit less compact than ours (7.5 ounces compared to our 4 ounces).
Top
view of the board: poti moved to lower left because not much space at
the right
due to output plugs
Whole
PA box: on-off switch on top, batteries at the bottom, input left,
outputs
right
Soldering-side
of the board: 4.5V-VDD and ground routed in thick non-isolated wires.